Embodiments of the invention relate generally to electric drive systems for hybrid and electric vehicles and, more particularly, to a vehicle propulsion system for hybrid and electric vehicles with one or more energy storage devices and one or more electromechanical devices and an optimized method of controlling operation of the vehicle propulsion system.
Purely electric vehicles use stored electrical energy to power an electric motor, which propels the vehicle and may also operate auxiliary drives. Purely electric vehicles may use one or more sources of stored electrical energy. For example, a first source of stored electrical energy may be used to provide longer-lasting energy while a second source of stored electrical energy may be used to provide higher-power energy for, for example, acceleration, with one or both the first and second sources being capable of being charged through regenerative braking.
Hybrid electric vehicles may combine an internal combustion engine and an electric motor powered by an energy storage device, such as a traction battery, to propel the vehicle. Such a combination may increase overall fuel efficiency by enabling the combustion engine and the electric motor to each operate in respective ranges of increased efficiency. Electric motors, for example, may be efficient at accelerating from a standing start, while combustion engines may be efficient during sustained periods of constant engine operation, such as in highway driving. Having an electric motor to boost initial acceleration allows combustion engines in hybrid vehicles to be smaller and more fuel efficient.
While propulsion system configurations for purely electric vehicles and hybrid electric vehicles have been developed to include multiple sources of electrical energy to increase energy or power density and multiple power sources to achieve desired propulsive output, incorporating these energy storage and power sources into a propulsion system increases the overall size, weight, and cost of the system. For example, to ensure a minimum level of performance will be maintained over the desired life of the vehicle, batteries are often oversized to reduce power and cyclic stresses. Also, overly aggressive thermal management controls are implemented to help reduce thermal stresses on the batteries. Both of these approaches increase the overall vehicle size, increase manufacturing costs, and increase the operating costs of the energy storage system.
Traditional energy storage units for hybrid and electric vehicles are designed and implemented with little control over the degradation rate of the energy storage units or batteries within the system. Known battery life prognosis is performed off-line using physics-based models to predict the rate of various individual degradation mechanisms. These experimental models may take into account solid-electrolyte interphase (SEI) resistance growth and capacity fade, chemical reaction paths for SEI growth, the onset of particle fracture due to high-rate charge/discharge, or the electrochemical state for a single duty-cycle of a battery. To date, however, known models do not predict the post-initiation crack propagation needed to correlate actual capacity fade with the experimental data and lack the predictive capability for arbitrary battery duty-cycles.
Further, the off-line life testing of battery technologies is typically performed in an accelerated manner that condenses many cycles into a much shorter period of time than the battery would experience during normal operation. As such, the empirical models developed using accelerated aging testing may not accurately account for the interactions between the calendar-related and cycling-related response of the battery in a real-time, real-world application.
In addition to the operation of energy storage units, the system efficiency of hybrid and electric power systems is also affected by the DC link voltage of the drive system. One known technique for determining the DC link voltage uses a comprehensive model to calculate a DC bus voltage that minimizes motor and inverter loss for a particular vehicle propulsion system configuration. Use of such a comprehensive model is time intensive and results in expensive hardware deployment. Moreover, such a method relies on the model's accuracy and is inevitably not robust to varying system components and operational modes. Another technique for determining a DC link voltage uses a motor system efficiency map to search for a voltage level with minimal loss. As this technique relies on a direct look-up table, noise on all input appears on the output voltage command. Moreover, the look-up table is static and does not take system dynamics into account. Thus, a sudden load change may cause unsatisfactory responsive performance on motor torque due to the latency of the voltage command. The comprehensive model likewise fails to respond satisfactorily to sudden load changes, typically adding a predetermined margin to the voltage command to accommodate any dynamic uncertainly. However, such a predetermined margin often produces an unsatisfactory response; since too large of a margin sacrifices system efficiency while too small of a margin will not meet the requested dynamic response.
As outlined above, known techniques for configuring a hybrid or electric propulsion system to operate with multiple energy storage sources and one or more power sources rely on experimentally determined models and static data that does not account for real-time, real-world system dynamics and operating conditions. Accordingly, use of these known techniques reduces the operating efficiency and fuel economy of the individual components of the propulsion system in addition to reducing the overall system efficiency.
Therefore, it would be desirable to provide an electric and/or hybrid electric propulsion system that improves overall system efficiency and optimizes the operation and lifespan of the energy storage units while permitting the prolusion system to be manufactured at a reduced cost.